Ultrasonic parking systems assist drivers with everyday parking maneuvers. A fully equipped ultrasonic parking system currently includes 12 ultrasonic sensors, 6 sensors each being situated in the front and the rear. In this system, objects at a distance of a few centimeters as well as of several meters may be detected. To achieve ranges of several meters and, at the same time, maintain the proximity measuring capability, an ultrasonic transmission and reception device must optimally utilize the available energy and at the same time hold the decay time to a minimum.
In an ultrasonic transmission and reception device, a high-frequency alternating voltage signal is generated as a transmission signal by a transmission circuit. This alternating voltage signal is applied during a transmission phase to an ultrasonic transducer, in order to transmit ultrasonic waves. Since the transmission capacity and, therefore, the range of an ultrasonic transmission and reception device are substantially a function of the voltage amplitude of the transmission signal, the voltage amplitude of the transmission signal is amplified by a transformer prior to being fed into the ultrasonic transducer. In a receiving phase, reflected ultrasonic signals are received by the ultrasonic transducer and tapped there by a reception circuit.
In the process, the transmission signal with the voltage amplitude amplified by the transformer is present at the input of the reception circuit during the transmission phase. Since this reception circuit is suited for evaluating very low amplitude echo signals, its proof voltage is limited. To prevent the reception circuit from being damaged, the reception circuit is protected with a series resistor, or else the components of the reception circuit are configured with a correspondingly high proof voltage.
However, these protective measures for protecting the reception circuit entail additional costs in the manufacture of the ultrasound transmission and reception device due to expensive or additional components.
An ultrasonic transmission and reception device according to the related art is shown in FIG. 7. When considering the circuit design, it becomes apparent that the circuit is optimized for proximity measuring capability only. A reception circuit 30 in this case is implemented by an invertingly connecting operational amplifier 31. The reception circuit is decoupled by a capacitor C1 in relation to an ultrasonic transducer 40 and the secondary coil 15 of a transformer 10. Operational amplifier 31 in this case is amplified by a first resistor R1 connected between the inverting input of operational amplifier 31 and the input of reception circuit 30, and by a second resistor R2 connected between the output of operational amplifier 31 and its inverting input as follows: v=−(R2/R1). Because of the requirement of the optimally short decay time, R1=Rs. Rs in this case is an internal resistance of ultrasonic transducer 40. The energy in the system after transmission is best dissipated via first resistor R1 and internal resistor Rs, and a high current flows into the input of operational amplifier 31 of reception circuit 30. During transmission, a current is fed into primary coil 14 of transformer 10. A current flows on the secondary side of transformer 10 in accordance with the number of windings of the transformer on primary side 11 and secondary side 12 of transformer 10. In the case of the resonance frequency of the transformer and in the transient oscillation state, the current is divided proportionately between internal resistor Rs of ultrasonic transducer 40 and of first resistor R1. Thus, half of the power output in the reception path is channeled off unnecessarily. Ultrasonic transducer 40 receives only half of the entire power output and therefore delivers only a sound pressure reduced by 3 dB.
An ultrasonic transmission and reception device is described in German Published Patent No. 10 136 628 B4.